1. Field of the Invention
The invention relates to a device for conveying a stream of sheets, in particular, a shingled or imbricated stream thereof, to a sheet-processing machine, having a conveyor table formed with suction openings extending therethrough and provided with a transport belt drivable revolvingly around it, a suction box disposed under the conveyor table and having vacuum applied thereto, the suction box being connected via the suction openings formed in the conveyor table to the underside of the transport belt.
In such devices, so-called suction belt tables, there is a general problem that the first and the last sheet must also be transported reliably. The act of conveying a shingled stream of sheets on the suction belt table may be divided, in this regard, into three phases.
The first phase may be characterized by the fact that during startup of the printing machine, the first sheet, which is pushed onto the suction belt from a sheet separating or singling device, is supposed to be sucked with as little slip as possible onto the perforated transport belt. In this first phase of machine operation, however, the suction force is basically reduced so that only a given region of the transport belt is covered by the first sheet and, thus, a great amount of extraneous air is sucked in, with the result that the suction effect is limited.
In the second phase, namely the conveying of the shingled stream of sheets, which somewhat corresponds to the stationary or steady state of the system, constant operating conditions prevail, for which reason this second phase can be classified as nonproblematic.
The third phase of conveying a shingled stream of sheets relates to the conveyance of the last sheet of the shingled stream of sheets. This last sheet is sucked over the entire length thereof onto the transport belt and is thus fixed significantly more strongly on the transport belt than the partly overlapping sheets of the shingled stream of sheets. In the sheet stream, the fact that the individual sheets overlap results in the sheets being in contact only over a fraction of the area thereof with the transport belt and being consequently subjected to a reduced sucking effect. If the last sheet, during transport thereof towards the front lays, remains subjected to the vacuum prevailing in the suction belt table, the leading edge of the last sheet can be pressed against the front lays, on the one hand, or the lateral alignment can be hampered considerably, on the other hand.
The published German Patent Document DE 44 16 289 A1 has disclosed a device for conveying a sheet stream, in particular, a shingled sheet stream, to a sheet-processing machine. A suction box provided in this publication is divided by profiles, which are arranged therein, into individual suction chambers so that an effective vacuum, which is not subject to the influence of leakage, may be provided even in those regions arranged farther removed from the suction source.